1. Field of the Invention
The present invention relates to a circuit device and a method of manufacturing the same, and more particularly relates to a circuit device in which large circuit elements are connected with solder and a method of manufacturing the same.
2. Description of the Related Art
With reference to FIGS. 10A to 11B, a method of manufacturing a conventional circuit device will be explained. Here, description will be given of a method of manufacturing a hybrid integrated circuit device in which a conductive pattern 108 and circuit elements are formed on a surface of a substrate 106. This technology is described for instance in Japanese Patent Application Publication No. 2002-134682.
As shown in FIG. 10A, first, a solder 109A is formed on a surface of the conductive pattern 108 formed on the surface of the substrate 106. The substrate 106 is, for example, a metal substrate made of metal such as aluminum. The conductive pattern 108 and the substrate 106 are insulated from each other by an insulating layer 107. The conductive pattern 108 forms pads 108A, 108B and 108C. A heat sink is fixed to an upper part of the pad 108A in a subsequent step. A small signal transistor is fixed to the pad 108B in a subsequent step. A lead is fixed to the pad 108C in a subsequent step. Here, the solder 109A is formed on each of surfaces of the pad 108A, which is a relatively large pad, and the pad 108C.
As shown in FIG. 10B, next, a small signal transistor 104C and a chip component 104B are fixed by use of a solder 109B. In this step, heating is performed until the solder 109B, which connects the transistor 104C and the like, is melted. Therefore, the solder 109A formed on each of the pads 108A and 108C in the preceding step is also melted.
As shown in FIG. 10C, next, the small signal transistor 104C and a predetermined conductive pattern 108 are connected to each other by use of a thin wire 105B.
As shown in FIG. 11A, next, the solder 109A previously formed on each of the pads 108A and 108C is melted to fix a heat sink 111 and a lead 101. Here, the heat sink 111 having a power transistor 104A mounted thereon is fixed onto the pad 108A with the previously formed solder 109A interposed therebetween. Furthermore, a desired conductive pattern 108 and the transistor 104A are connected to each other by use of a thick wire 105A.
As shown in FIG. 11B, a sealing resin 102 is formed so as to cover the circuit elements and the conductive pattern 108 which are formed on the surface of the substrate 106. By the above steps, a hybrid integrated circuit device 100 is manufactured.
However, as shown in FIGS. 12A to 13B, in a step of fixing the chip component 104B and the like by heating and melting the solder 109B, there is a problem of sink in the solder 109A. FIG. 12A is a plan view of the substrate 106 in which sink occurs. FIG. 12B is a cross-sectional view of FIG. 12A. FIG. 12C is an enlarged cross-sectional view of a portion where the sink occurs. Moreover, FIGS. 13A and 13B are views showing in detail the boundary between the solder I 09A and the pad 108A.
As shown in FIGS. 12A and 12B, “sink” means a phenomenon that the solder 109A formed on the entire surface of the pad 108A is accumulated on one side or the other. Particularly, the pad 108A on which the heat sink 111 is fixed is formed to have a large rectangular shape with a length of 9 mm or more on a side, for example. Therefore, compared with other portions, a large amount of solder is attached to the upper part of the pad 108A. Accordingly, a large surface tension acts on the melted solder 109A. Thus, sink of the solder occurs.
Particularly, the sink of the solder 109A frequently occurs in the step of fixing the chip component 104B by use of the solder 109B. Specifically, in this step, solder paste applied to the small pad 108B is heated and melted to fix the chip component 104B and the like. By the heating and melting in this step, the solder 109A previously formed on the surface of the relatively large pad 108A is also remelted. Since the remelted solder 109A includes no flux unlike the solder paste, a large surface tension acts thereon. Therefore, sink occurs in the remelted solder 109A.
As shown in FIG. 12C, generation of an alloy layer 110 between the pad 108A and the solder 109A is one of the causes of the occurrence of the sink. When the solder paste is attached to the upper part of the pad 108A and heated and melted, an intermetallic compound is formed, which is made of copper that is a material of the pad 108A and tin that is a material of the solder. In FIG. 12C, a layer made of the intermetallic compound is indicated by the alloy layer 110. To be more specific, a thickness of the alloy layer 110 is about several μm, and an intermetallic compound having a composition of Cu6Sn5 or Cu3Sn is formed. This alloy layer 110 has poor solder wettability compared with copper that is the material of the pad 108A. Accordingly, formation of the alloy layer 110 having the poor solder wettability causes the sink of the solder. In the below description, the alloy layer made of copper and tin is called a Cu/Sn alloy layer.
Furthermore, activation of an interface between the alloy layer 110 and the solder 109A by melting of the alloy made of copper and tin into the solder 109A is also one of the causes of the occurrence of the sink described above.
FIG. 13A is a cross-sectional view of the substrate 106 in which the above-described sink occurs. FIG. 13B is a SEM (scanning electron microscopy) image of a cross section of the boundary between the pad 108A and the solder 109A.
As shown in FIG. 13B, on the boundary between the pad 108A and the solder 109A, the alloy layer 110 made of copper and tin is generated. As described above, since the solder 109A is melted more than once, formation of the alloy layer 110 which is as thick as about 5 μm or more, for example, induces sink. Moreover, the intermetallic compound made of copper and tin is formed at a high rate. Thus, activation of the boundary between the solder 109A and the pad 108A is also the cause of occurrence of the sink. Furthermore, the intermetallic compound is formed not only in the boundary therebetween but also in the solder 109A, for example.
Furthermore, although not clearly shown in the SEM image, a number of hemispherical protrusions, each of which has a size of about 5 to 10 μm, for example, and is made of the intermetallic compound, are formed on the entire upper surface of the alloy layer 110. The alloy layer 110 has a relatively smooth surface. The formation of the protrusions reduces an interface resistance on the upper surface of the alloy layer 110 and leads to a situation where the solder 109A is likely to slip on the surface. Thus, occurrence of the sink described above is promoted.
Meanwhile, in consideration of the environments, lead-free solder has recently been used. If the lead-free solder is used as the solder 109A, the problem of the sink described above occurs more prominently. This is because the lead-free solder contains more tin than lead eutectic solder does. To be more specific, a proportion of tin contained in a general lead eutectic solder is about 60 wt %. On the other hand, a proportion of tin contained in the lead-free solder is about 90 wt %, which is relatively large. Furthermore, when the lead-free solder is melted, a temperature is higher than that on the occasion when the lead eutectic solder is melted. This is also the cause of formation of the thick alloy layer 110. To be more specific, when the lead eutectic solder is melted, the temperature is about 200° C. Meanwhile, when a lead-free solder having a composition of Sn-3.0Ag-0.5Cu, for example, is melted, the temperature is about 240° C. As described above, the higher the melting temperature is, the more the chemical reaction is accelerated. Thus, the alloy layer 110 having poor wettability is formed to be thicker.
When the sink of the solder 109A occurs, the pad 108A and the circuit element are not connected to each other in the portion where the sink occurs. Therefore, thermal resistance in the portion where the sink occurs is increased. Furthermore, strength of a solder connection part is lowered by the occurrence of the sink. Thus, connection reliability of the solder connection part with respect to a change in temperature is lowered.